PDDL and other languages..

1
PDDL and other languages..

• Lee McCluskey
• Department of Computing and
• Mathematical Sciences,
• The University of Huddersfield

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Background

• Related to AI Planning there are several kinds of
  knowledge that is required declaratively
• Domain/environment
• Planning heuristics
• Task (input)
• Plans (output) ? actions/activities
• Execution - resources

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PDDL

• A language convention for describing DOMAIN
  DYNAMICS (?and nothing else?) which has succeeded
  through its use in the AIPS 98,00 and 02 planning
  competitions.
• Its aim of encouraging sharing of planning
  problems and algorithms has to some degree been
  achieved.

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PDDL - form

• PDDL?s syntax is LISP like. A domain (model)
  definition is structured into components by
  Keywords e.g. ?constants? ?actions? etc. The
  most important components are the set of actions.
• A special keyword is ?requirements? which tells
  a process which blend of PDDL features are used
  in the domain definition. So we have a family of
  languages to suit planners with different
  capabilities.

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PDDL - Semantics

• The basic requirement in PDDL is ?strips? which
  indicates the underlying semantics of the
  language ?
• worlds are considered as sets of situations
  (states), where each state is specified by
  stating a list of all predicates that are true.
  States are changed instantaneously into new
  states by actions which change the truth value of
  predicates. Actions have preconditions and
  effects under the default persistence
  assumption.. Etc?

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PDDL - Where is the Semantics?

• The semantics of PDDL v1.2 (used for ?98
  competition) are informal and appear to be
  distributed among
• the pre-existing languages/systems ? strips,
  ucpop
• The v1.2 manual
• The language processors (solution checker)
• LISP
• somewhere else??

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PDDL

• Examples ..
• ?The rule is that action definitions are not
  allowed to have effects that mention predicates
  that occur in the implies field RHS of an
  axiom? (p13)
• ?An action definition must have an effect or an
  expansion but not both? (p8)

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PDDL - v2.1

• Extensions ? different handling of numeric
  quantities, addition of durative actions
• Left out ? HTN actions (apparently no-one had
  used them!)
• BUT ? attempted to give a formal semantics to the
• language

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Is PDDL a (good) modelling language??

• Fox and Long in the v2.1 manual describe it
  explicitly as one. Although not much discussed,
  PDDLv1.2 actually provides modelling features..
• timeless - predicates (static factual knowledge)
• domain-axioms ? written as L-R rules that form
  invariants on situations
• expansion ? allows encapsulation of actions in
  an HTN fashion
• extends ? allows some modularisation - one can
  import previously written components.

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Is PDDL a modelling language??

• But
• PDDL was designed to reflect current languages
  and their underlying assumptions. It was NOT
  designed with a model building method in mind OR
  with many ?pragmatic? feature which make building
  easier.
• It is a ?machine code? rather than a language for
  human use!

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Role of PDDL in the Semantic Web?

• . One can imagine having planning services around
  the web ? one supplies the problems domain
  model in extended-PDDL and invokes the planning
  service.
• Extensions
• Marked up (XML/RDF/RDFS (?)) version of PDDL
• Language for expressing advice / heuristics
• Service analyses the domain model and configures
  a planner to solve the problems

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Future ? develop ontologies for Planning

• Ontologies are explicit specifications of a
  conceptual model for sharing the understanding of
  a particular domain.
• Some ontologies for planning concepts have been
  created ? e.g. PLANET (Blythe) and SPAR (Tate).
  They are deemed essential for on-line agent
  communication between agents involved in planning
  (but promise multiple benefits eg in the KA
  process).

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Future ? develop ontologies for Planning

• Both
• - planning-oriented
• AND
• - planning-application-oriented
• ontologies need to be developed.

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Future ? my vision?

• Timely maturing of 4 research areas ?
• Semantic Web
• KE ? knowledge sharing and re-use
• Planning language conventions
• Planning KE
• Can be combined to solve the biggest problems in
  AI planning currently ? lack of Accessibility and
  Usability of the technology